1. The problem statement, all variables and given/known data Specify and explain whether the following is an inertial or non-inertial observer: An observer is placed on a rock between Andromeda and the Milky Way. 2. Relevant equations N/a 3. The attempt at a solution So here is my understanding, the observer would be situated within an inertial reference frame as it is not specified whether there is an acceleration relative to either of the galaxies, my curiosity is that since the Andromeda galaxy is traveling towards the Milky Way does this mean that the observer is in fact accelerating relative to Andromeda? Or is the observer still stationary relative to Andromeda? Bit confused, I'd be more than grateful for any clarification.